Preventing circumvention of path-based MAC via links

AppArmor can be circumvented via hardlinks in the standard POSIX security model. However, the kernel now includes the ability to prevent this vulnerability, without needing the patches distributions like Ubuntu have applied to their kernels as workarounds.

Implementation Status

Not all the packages work out-of-the-box, but it is a work in progress. If you know how to build profiles yourself you shouldn't have too many problems.
Also there is an AUR kernel
which includes apparmor specific patches from Ubuntu's launchpad.

AppArmor Packages

Kernel Configuration

Here is configuration of ArchLinux kernel which enables AppArmor (just FYI, you do not need to touch it):

CONFIG_SECURITY_APPARMOR=y
CONFIG_SECURITY_APPARMOR_BOOTPARAM_VALUE=0
# CONFIG_DEFAULT_SECURITY_APPARMOR is not set

However, integration of AppArmor into the 2.6.36 kernel is not quite complete. It is missing network mediation and some of the interfaces for introspection. See here for details. There are compatibility patches that can be applied to every recent kernel to reintroduce these interfaces. The patchset is pretty small and should be applied if you decide to use AppArmor. (Note: the patchset for 2.6.39 works with Kernel 3.0.x)

Bootloader Configuration

Enable

To test profiles, or enforce the use of AppArmor it must be enabled at boot time. To do this add apparmor=1 security=apparmor to the kernel boot parameters.

After reboot you can test if AppArmor is really enabled using this command as root:

# cat /sys/module/apparmor/parameters/enabled
Y

(Y=enabled, N=disabled, no such file = module not in kernel)

Disable

AppArmor will be disabled by default in Arch Linux, so you will not need to disable it explicitly until you will build your own kernel with AppArmor enabled by default. If so, Add apparmor=0 security="" to kernel boot parameters.

More Info

AppArmor, like most other LSMs, supplements rather than replaces the default Discretionary access control. As such it's impossible to grant a process more privileges than it had in the first place.

Ubuntu, SUSE and a number of other distributions use it by default. RHEL (and it's variants) use SELinux which requires good userspace integration to work properly. People tend to agree that it is also much much harder to configure correctly.

Taking a common example - A new Flash vulnerability: If you were to browse to a malicious website AppArmor can prevent the exploited plugin from accessing anything that may contain private information. In almost all browsers, plugins run out of process which makes isolating them much easier.

AppArmor profiles (usually) get stored in easy to read text files in /etc/apparmor.d

Every breach of policy triggers a message in the system log, and many distributions also integrate it into DBUS so that you get real-time violation warnings popping up on your desktop.